Digital subscriber line (DSL) technology is a high speed transmission technology for data transmission over telephone twist pair, that is, unshielded twist pair (UTP). The DSL technology includes asymmetrical digital subscriber line (ADSL), very-high-bit-rate digital subscriber line (VDSL), single-pair high-bit-rate digital subscriber line (SHDSL), asymmetrical digital subscriber line 2 (ADSL2), asymmetrical digital subscriber line 2plus (ADSL2plus), and very-high-bit-rate digital subscriber line 2 (VDSL2), with xDSL being a general designation for the above DSL technology.
Within the frequency range used by an xDSL, a wide range of radio frequency waves exist, these waves may be used in a broadcasting station, for radio communication, and the like. Radio frequency bands in China are specified as follows: 120-300 kHz is used for long-wave amplitude modulation broadcast; 525-1605 kHz is used for medium-wave amplitude modulation broadcast; and 3.5-29.7 MHz is used for short-wave amplitude modulation broadcast, single sideband communication, and amateur radio.
All the frequencies used by the radio services overlap more or less with the frequencies used by xDSL services. When radio waves transmitted by the radio services are coupled to an xDSL, and have the same frequency as that of some of sub-carriers of the xDSL, the transmission of the xDSL sub-carriers are affected, which causes instability of the xDSL services and generates an error or disconnection. Such radio interference is referred to as RFI.
RFI band specified by ITU-T is a measure for shielding RFI proposed by ADSL2plus and VDSL2. If the band range of RFI interference is known, the DSL sub-carriers at these frequency bands may be shielded. In this manner, RFI interference at these frequencies will not cause an error in a DSL line, or will not affect the stability of the DSL line. In addition, a DSL transceiver may also reduce transmission power at these RFI frequencies, thus avoiding the interference of the DSL services to broadcasting services.
Such technologies as RFI band only provide a method for solving RFI when the RFI frequency is known. But how to obtain the RFI frequency is not mentioned. Generally, the RFI frequency can only be acquired through operator's experience or through investigation. The method does not work if the RFI frequency is not known.